Wireless Energy Transfer
Inductive coupling is used in a number of wireless energy transfer systems, such as a cordless electronic toothbrush, or vehicle batteries. In coupled inductors, such as transformers, a source, e.g., a primary coil, generates energy as an electromagnetic field, and a sink, e.g., a secondary coil, subtends that field such that the energy passing through the sink is optimized, e.g., the energy generated by the sink is as similar as possible to the energy of the source. To optimize the energy, a distance between the source and the sink should be as small as possible, because over greater distances the inductive coupling method is highly ineffective.
Resonant Coupling System
FIG. 1 shows a conventional resonant coupling system 100 for transferring energy from a source 110 to a sink 120. In resonant coupling, two resonant electromagnetic objects, i.e., the source and the sink, interact with each other under resonance conditions.
A driver 140 inputs the energy into the source to form an oscillating electromagnetic field 115. The excited electromagnetic field attenuates at a rate with respect to the signal frequency at the driver or self resonant frequency of the source and sink in a resonant system. However, if the sink absorbs more energy than is lost during each cycle, then most of the energy is transferred to the sink. Operating the source and the sink at the same resonant frequency ensures that the sink has low impedance at that frequency, and that the energy is optimally received.
The energy is transferred, over a distance D, between resonant objects, e.g., the source has a length L1 and the sink has a length L2. The driver connects a power provider to the source. The sink is connected to a power consuming device, e.g., a resistive load 150. Power is supplied by the driver to the source, transferred wirelessly and non-radiatively as energy from the source to the sink. The rate of energy transfer powers the load. The wireless non-radiative energy transfer is performed using the field 115, e.g., the electromagnetic field or an acoustic field of the resonant system. For simplicity of this description, the field 115 is an electromagnetic field. During the coupling of the resonant objects, evanescent fields 130 are propagated between the source and the sink.
However, the resonant coupling transfers energy from the source to the sink over a mid-range distance, e.g., a few times of the resonant frequency wavelength, is inefficient when the distance becomes larger. It is thus desirable to extend the range of efficient wireless energy transfer.